In the wake of terror attacks on September 11th, the threat of terrorists using a Radiation Dispersal Device (RDD), commonly referred to as a “dirty bomb” or an improvised nuclear device (IND) is a frightening possibility. In fact, several government agencies have concluded that terrorist groups like Al'Qa'ida are actively seeking to acquire radionuclear devices for use against U.S. citizens and interests, both here in the United States and abroad.
If this threat is realized, federal, state and local authorities must be prepared. An important aspect of any emergency management plan following such an event involves the safety and monitoring of: first responders, health-care workers, and citizens that are exposed to radiation resulting from the radiological or nuclear device.
While radiation exposure of first responders and health-care workers may at least partially be monitored using traditional radiation detecting devices, monitoring exposure of potentially tens of thousands of citizens presents a more difficult problem.
After removable contamination has been eliminated, there may be a need for personal external dosimetry monitoring for individual members of the public as well as large numbers of workers. Site restoration could be a lengthy project, and to minimize disruption to society it may be necessary to allow inhabitants to have access to certain areas of the city before cleanup is complete. For example, allowing citizens to pass through transit centers, thoroughfares, or certain areas of buildings could permit government operations, commerce, uniting of families, routine medical treatments, etc. As an individual moves through a contaminated city, the locations visited and time spent at them would vary, and a personal dosimeter would allow tracking of the accumulated dose. Such dose measurements could reduce reliance on models and estimates and avoid unnecessary area denial time. Unlike cleanup at decommissioned facilities where the public could be excluded with little cost to society, in an urban environment, time is of the essence and the cost of exclusion may be greater than the benefit avoiding exposure to a relatively low radiation dose. After cleanup, personal dosimetry could boost public confidence that their external dose is below acceptable maximums and that the final cleanup was effective. A need exists for a personal radiation detection device (i.e. dosimeter) that is small and easily carried, provides field-readable exposure data, is cost effective and is capable of providing dosimetry of record over a wide dose range. A dosimeter in the familiar format of a credit card or identification card that can be readout at stations similar to transit or bank card readers would meet these needs.
The Citizen's Dosimeter fills a technology gap among other currently available dosimeter options. For example, the SIRAD ‘casualty dosimeters’ are designed to be carried to identify medically significant exposures for triage. Alarming, interdiction electronic radiation monitors are expensive ($100+) and not widely accepted for dose of record in the United States. The approximately 85,000 personal radiation dosimeters stockpiled by states for emergency use and those currently used for monitoring radiation worker dose of record are not readable in the field. In contrast, the Citizen's Dosimeter would be used in the aftermath of an event, sensitive to the entire range of radiation levels spanning public and worker dose limits as well as casualty levels, field readable while providing dose of record, and inexpensive at about $20 a piece. See Klemic, G., Bailey, P., Miller, K., Monetti, M. External radiation dosimetry in the aftermath of radiological terrorist event, Rad. Prot. Dosim, in press;
Several radiation measurement technologies currently exist including TLD dosimeters, OSL dosimeters, electronic dosimeters, and quartz or carbon fiber electrets.
Thermoluminescent Dosimeter (TLD) badges are personal monitoring devices using a special material (i.e. lithium flouride) that retains deposited energy from radiation. TLD badges are read using heat which causes the TLD material to emit light which is then detected by a TLD reader (calibrated to provide a proportional electric current). One disadvantage of TLD badges is that once read, the signal of the device is erased or zeroed out. Furthermore, it takes between approximately 20-30 seconds to obtain the reading. In dosimetry programs which use TLDs for dosimetry of record, the dosimeters are returned to a processing laboratory for readout. Optically Stimulated Luminescence (OSL) badges use an optically stimulated luminescent material (OSLM) (i.e. aluminum oxide) to retain radiation energy. Tiny crystal traps within the OSL material trap and store energy from radiation exposure. The amount of exposure is determined by shining a light of one color (i.e. green) on the crystal traps and measuring the amount of light of another color (i.e. blue) emitted. Alternatively, pulsed light stimulation can be used to differentiate between the stimulation and emission light [there is a patent for this, held by Stephen McKeever, See, U.S. Pat. Nos. 5,892,234 and 5,962,857, issued to McKeever et al. Unlike TLD, OSL systems provide readouts in only a few seconds and provides multiple readouts since only a very small fraction of the exposure signal is depleted when readout. In current dosimetry programs, for dosimetry of record, based on OSL and TLD dosimeters, the dosimeters are returned to a processing laboratory for readout.
One of the problems with state of the art OSL systems is that the filters used within the dosimeters are too thick for use in a thin, credit card sized dosimeter. For more information on OSL materials and systems, see, U.S. Pat. No. 5,731,590 issued to Miller; U.S. Pat. No. 6,846,434 issued to Akselrod; U.S. Pat. No. 6,198,108 issued to Schwietzer et al.; U.S. Pat. No. 6,127,685 issued to Yoder et al.; U.S. patent application Ser. No. 10/768,094 filed by Akselrod et al.; all of which are hereby incorporated by reference in their entireties. See also, Optically Stimulated Luminescence Dosimetry, Lars Botter-Jensen et al., Elesevier, 2003; Klemic, G., Bailey, P., Miller, K., Monetti, M. External radiation dosimetry in the aftermath of radiological terrorist event, Rad. Prot. Dosim, in press; Akslerod, M. S., Kortov, V. S., and Gorelova, E. A., Preparation and properties of Al2O3:C. Radiat. Prot Dosim 47, 159-164 (1993); and Akselrod, M. S., Lucas, A. C., Polf, J. C., McKeever, S. W. S. Optically stimulated luminescence of Al2O3:C. Radiation Measurements, 29, (3-4), 391-399 (1998), all of which are incorporated by reference in their entireties.
Electronic dosimeters are battery powered, have a digital readout, and audio or vibrating alarming capabilities. These instruments record cumulative dose and have the added advantage of giving real-time dose rate information to the wearer. For routine occupational radiation settings in the U.S. electronic dosimeters are mostly, but not strictly, used for access control and not for dose of record. A number of cities and states issue electronic dosimeters to HAZMAT teams as part of their emergency response plans. There are presently tens of thousands of electronic dosimeters deployed for homeland security purposes, however, electronic dosimeters are impractical for mass use as personal (citizen's) dosimeter due to their high cost.
Quartz or carbon fiber electrets are cylindrical electroscopes where the dose is read by holding it up to the light and viewing the location of the fiber on a scale through an eyepiece at one end. A manually powered charger is needed to zero the dosimeter. The quartz fiber electret is an important element of many state emergency plans. For example, some plans call for emergency responders to be issued a quartz fiber electret along with a card for recording the reading every 30 minutes, as well as a cumulative dosimetry badge or wallet card. While they are specified for use in nuclear power plant emergencies, the NRC does not require them to be NVLAP accredited, only that they be calibrated periodically.
A need exists for a personal radiation detection device (i.e. dosimeter) that: is small and portable (i.e. card-card size), provides real time exposure data, and is cost effective and is capable of providing dosimetry of record.